There's way too many things that can be changed requiring other things to be changed, so making something like this would take a very long time and would be pretty complex.
As far as shields, it's not too bad:
Buildings when constructed don't start with full shields, so even if you give them a max shield value they start at 0/max, and if they don't have shield recharge set they'll stay at 0 until something else recharges them.
Now, having shields also means that a weapon now has to do damage to shields, and since the game knows it's doing damage to shields, it needs to draw the shield impact, and for that it needs the unit/structure to have the shield mesh. So if you didn't set one it'll try looking for it and if it doesn't exist, game goes oops
If you have questions, feel free to post and myself and others can try to answer them as best as we can, but there probably won't be a complete listing of how to change everything for a while.
